Gap junctions and their structural component, connexins, are considered to be highly important in the control of cell proliferation and neoplasia. Thus, understanding the molecular mechanism of the regulation of gap junctional communication (GJC) by growth factors and tumor promoters is of paramount importance and is critical for designing future cancer preventive or therapeutic strategies. Particularly interesting and of potential clinical importance is the observation that connexin43 (Cx43), one of 12 members of the connexin family, functions as a tumor suppressor gene in several models of human cancer. The tumor suppressive actions of Cx43 in the present proposal will focus specifically on prostate cancer. The current research proposal is also focused on the mechanism of the platelet-derived growth factor (PDGF) signaling pathway leading to the closure of the gap junctional channels composed of Cx43. The presently accepted mode is phosphorylation of Cx43 although identification of the responsible kinase or the phosphorylation sites remains unknown. Several of our studies, however, suggest that phosphorylation of Cx43 is not sufficient for and may not be involved in PDGF-induced GJC blockade. The first set of proposed studies will confirm the role of Cx43 phosphorylation and based on results, a second set of experiments is designed to identify the phosphorylation sites of Cx43 by utilizing a series of deletion and point mutants of Cx43. Subsequent characterization of the responsible kinase will be accomplished by several in vitro assays which will ultimately lead to its cloning and sequencing. Since protein-protein interaction is also suggested by our studies, attempts will be made to identify such Cx43- associated proteins. Finally, by comparing the effect of wild-type and mutant Cx43, our studies will determine whether the tumor-suppressive action of Cx43 in human prostate cancer is dependent on the establishment of GJC. Such a finding will strongly indicate novel cellular functions for Cx43.